Devices providing adjustable optical power have been proposed for use as corrective lenses in prescription eyeglasses. Eyeglasses equipped with adjustable lenses have significant potential in the developing world, where a single model of eyeglasses with an appropriate optical power range can be used to correct refractive error in a large section of a population. Such a model of eyeglasses could be deployed without the use of eye care professionals and so bring vision correction to many populations who are currently underserved. In an article by Zhang, M et al., “Self correction of refractive error among young people in rural China: results of cross sectional investigation”, BMJ 2011; 343:d4767, it is demonstrated that a model of adjustable eyeglasses used as a self-correction device and applied using a protocol supervised by trained lay persons can achieve good visual outcomes for young people aged 12-18. A model of eyeglasses equipped with adjustable lenses can be used to correct presbyopia amongst adults in both developing and developed world situations, assuming that model can provide a modest range of positive optical power. This allows the wearer to adjust the power to suit various tasks such as reading, using a personal computer or other close work.
In order to function effectively adjustable lenses must provide a satisfactory level of optical performance, which will depend on the context in which the lenses are applied. For an application such as adjustable power corrective lenses in eyeglasses the device should provide good quality correction with low higher-order aberrations (preferably less than a quarter wavelength of 633 nm light for a pupil diameter of 4 mm) over the region of required optical correction (which may or may not be the entire lens), good transmission, low distortion, and low variation of mean spherical power and any unwanted astigmatism over a range of viewing directions through the lens (corresponding to the eyeball rotating to look through different parts of the lens).
The adjustable lenses should provide a range of corrective power useful for the intended the application. For example, for presbyopic applications a range of 2 to 4 dioptres may be appropriate. The lenses should not be excessively thick or heavy as this will have a negative effect on the appearance of eyeglasses and may also make a pair of eyeglasses featuring such lenses uncomfortable to wear.
It is known that a lens with adjustable power may be realised by employing two separate optical elements which may be moved relative to each other. For example, it is well known in elementary optics that the combination of two thin lenses of power P1 and P2 arranged such that their optical surfaces are substantially perpendicular to an optical axis and aligned so that the lens centres of the thin lenses lie on the optical axis has an overall optical power P given by the equationP=P1+P2−P1P2t,  (1)where t is the distance between the lens centres as measured along the viewing axis. Thus, by altering the distance t between the thin lenses a variation in the overall optical power of the system may be achieved. This principle is used to good effect in many optical devices. For example, the basic principle underlies the focusing elements in some simple camera lens designs. However, for some applications the requirement of an adjustable distance between two lens elements along an optical axis is undesirable as it will typically result in a system that has considerable total thickness, as measured along the optical axis from the front of the first lens to the back of the second lens. A large thickness may be a problem if limited space is available or in applications such as eyewear to correct for refractive error, where thin lenses are generally regarded to be cosmetically desirable as well as lighter and more comfortable to wear.
In the prior art work U.S. Pat. No. 3,305,294 by Alvarez an approach is described where two separate, specially shaped optical plates are moved relative to each other along an axis perpendicular to a central viewing axis along which the optical elements are arranged. In the preferred embodiment of U.S. Pat. No. 3,305,294 the optical device is composed of two refracting optical plate elements which are generally perpendicular to a central viewing axis. The variation of thickness of each of the plates is described by a cubic polynomial of Cartesian (x,y) coordinates in a plane perpendicular to the central viewing axis as shown schematically in FIG. 1. The variation in thickness is complementary in that when the plates are placed in a neutral position (defined as where the origin of the cubic thickness profiles of each of the plates lies on the optical axis) the cubic polynomial terms for the two plates have the same magnitude as each other but opposite sign. This complementary design of thickness variations results in a device that provides a variable amount of optical power when the plates are aligned together and slid relative to each other along the designated axis and viewed along a suitably arranged axis. Whilst achieving a reasonably thin system of lenses, the overall performance of optical devices based on the approach described in U.S. Pat. No. 3,305,294 is limited. This is particularly noticeable for applications where the actual viewing axis deviates in direction from the central viewing axis resulting in significant and unwanted variation in mean spherical optical power and astigmatism. This is described in more detail below.
In the prior art work U.S. Pat. No. 3,583,790, by Baker, a similar approach to U.S. Pat. No. 3,305,294 is described except the thickness variation of the optical plates are described by a fifth order polynomial function. As in U.S. Pat. No. 3,305,294 the variations in thickness of the two optical plates when in the neutral position are equal in magnitude and opposite in sign. The use of a quintic polynomial to describe thickness variation allows for better control of aberrations and superior optical performance for certain applications. However, the performance of the lens is again not well suited to applications where viewing through a wide range of angles is required, such as is the case for corrective lenses in eyeglasses.
U.S. Pat. No. 7,338,159 by Spivey discloses a technique for improving the performance of lenses based on optical elements moved relative to each other. The approach taken here again is to incorporate extra terms in the polynomial expression for the profiles of the optical plates with the goal of improving the optical performance of the resulting adjustable lenses.
U.S. Pat. No. 7,717,552 also by Spivey presents adjustable focus eyeglasses that make use of adjustable power lenses based on two elements with a pivot adjustment. This alternative mode of actuation results in a different form for the thickness variation of the individual elements, but the same principle is retained as with the other prior art work. This approach adds significant mechanical complication to the resulting design of the adjustable glasses.
U.S. Pat. No. 5,644,374 by Mukaiyama discloses a variable focus eyesight correction apparatus of a similar arrangement to Alvarez and Spivey, but states that contours of optical power of the lenses should be linear and parallel to principal meridians of the lenses.